The invention finds application in the field of digital data delivery over cable TV systems and possibly in cellular and satellite communications systems.
In the emerging field of digital data delivery over media such as hybrid fiber coaxial cable systems of cable television plants and wireless modems etc. there exists the problem of backward compatibilty. Some existing systems have time division multiplexed (TDMA) cable modems or other types of TDMA modems already installed, but newer technology makes faster TDMA modems possible and also makes synchronous code division multiplexed modems possible. It is desirable to be able to use these newer modems in existing systems without rendering the older modems obsolete or requiring them to operate on a different frequency band (hereafter referred to as a physical channel). For example, in all-TDMA systems, slower symbol rate modems such as DOCSIS 1.0 modems exist which can only transmit at a maximum symbol rate of 2.56 million symbols per second. However, faster TDMA modems are now available that can transmit TDMA bursts at 5.12 million symbols per second. Thus, a need has arisen for a method of allowing TDMA modems with different maximum symbol rates to coexist and function on the same shared transmission medium.
As another example of this problem, consider the existing DOCSIS digital data delivery systems for delivery of digital data over cable TV systems. In these prior art DOCSIS 1.0 systems, digital data delivery is made across the hybrid fiber/coax media of cable television systems bidirectionally using TDMA only at a maximum symbol rate of 2.56 Msymbols/sec. These systems used frequency division multiplexing to keep the digital data separate from the cable television signals. Multiple different physical frequency channels are used within the band of frequencies used for digital data delivery. Within each frequency channel, multiple DOCSIS 1.0 modems transmitted their data by modulating it onto a carrier having the same center frequency as the carrier used by all the other modems assigned to the same channel. Within each channel, time division multiple access multiplexing or TDMA was used to keep the data from each of the plurality of different DOCSIS 1.0 modems separate for transmission upstream to the CMTS (CMTS stands for Cable Modem Termination System and is the headend cable modem to which all remote unit cable modems transmit or head end.
This system works well, however, a movement is now underway to define a new national standard where synchronous code division multiplexed communication of digital data across cable television networks can be accomplished. The advantages of such a system are numerous including increased privacy, the ability of all modems with upstream traffic that have been awarded bandwidth to transmit at once, etc. This new proposed standard is publicly available as the IEEE 802.14a standard entitled xe2x80x9cHigh-capacity physical layer specification, Draft 1, Revision 3xe2x80x9d which was published Mar. 30, 1999 and will hereafter be referred to as the advanced PHY specification and modems which comply with it will be referred to as advanced PHY modems or advanced PHY transceivers. Although the IEEE 802.14a unapproved standards draft is in flux, its current state will be used for purposes of explaining this invention. advanced PHY modems currently must be capable of transmitting TDMA bursts at a faster maximum symbol rate than the DOCSIS 1.0 modems as well as to transmit SCDMA bursts. The advanced PHY proposal thus defines two modes of operation: (1) a TDMA mode which is defined both under the advanced PHY and the DOCSIS 1.0 standards; and (2) an SCDMA mode which is defined in advanced PHY only. An apparatus for transmitting digital data upstream using SCDMA with a chip clock rate derived from the arbitrary clock rate of an MCNS (MCNS is the acronym for a consortium of cable operators that backed the standardization of the DOCSIS specifications that define standards digital cable modems have, to meet in DOCSIS cable systems) or IEEE 802.14 downstream is defined in U.S. patent application Ser. No. 09/074,036, filed May 6, 1998 entitled APPARATUS AND METHOD FOR SYNCHRONIZING AN SCDMA UPSTREAM OR ANY OTHER TYPE UPSTREAM TO AN MCNS DOWNSTREAM OR ANY OTHER TYPE OF DOWNSTREAM WITH A DIFFERENT CLOCK RATE THAN THE UPSTREAM, which is hereby incorporated by reference. An earlier PCT publication WO97/08861 published Mar. 6, 1997 teaches the details of SCDMA transmitters and receivers for transmitting SCDMA frames on cable television systems, and is hereby incorporated by reference as are all the prior art publications incorporated by reference into this PCT publication including PCT publication WO97/34421, published Sep. 18, 1997 teaching methods and apparatus for using SCDMA to establish virtual links for use in transmitting ATM cells. A method of two dimension interleaving of data between minislots and spreading codes for the SCDMA upstream in the advanced PHY SCDMA bursts is taught in U.S. patent application Ser. No. 09/152,643, filed Sep. 14, 1998, which is hereby incorporated by reference. A method of using a bank of filters for excision of narrow-band interference to SCDMA signals is taught in U.S. patent application Ser. No. 09/152,645, filed Sep. 14, 1998, which is hereby incorporated by reference.
The downstream in MCNS systems for digital data delivery through cable TV systems breaks MAC layer packets down into MPEG packets. These are 64-QAM or 256-QAM modulated and sent as a continuous stream after FEC encoding. The upstream SCDMA transmitters in all the remote units derive their chip clocks from the downstream master clock and all transmit their SCDMA multiplexed data upstream in frames on the same frequency using the same chip clock. Alignment of the frame boundaries at the CMTS is achieved by doing a frame alignment offset calculation prior to carrying out the ranging process described in the PCT publications cited above to speed up the ranging process. The offset calculation determines the degree of offset of the minislot counter in each remote unit (hereafter RU) from the upstream minislot counter in the CMTS. This offset calculation is done using the timestamp messages normally sent in the downstream by sampling a local kiloframe counter in the RU each time a downstream sync message is received and performing a particular mathematical calculation.
While the advanced PHY SCDMA upstream is better, there is the problem of backward compatibility of the CMTS needed to implement advanced PHY transmissions with older systems populated by DOCSIS 1.0 TDMA only modems. If a CMTS head end apparatus is installed in a cable television system populated by a mixture of DOCSIS 1.0 TDMA only modems and the newer advanced PHY modems which can transmit in either TDMA or SCDMA, the older 1.0 TDMA only modems will not successfully communicate with the CMTS if they transmit on the same frequency as the newer modems and will have to operate on a different frequency band. Thus, a need has arisen for a new CMTS structure and process which can receive and process bandwidth requests and accomodate transmissions from both DOCSIS 1.0 TDMA only modems as well as advanced PHY TDMA or SCDMA modems on the same frequency channel and without requiring any changes to the DOCSIS 1.0 TDMA modems.
The basic idea underlying the process and apparatus of the invention is to provide a method by which multiple upstream channels having different symbol rates and/or multiplexing types can coexist on the same system and be transmitted using the same center frequency or the same PHYSICAL channel. For example, two sets of TDMA modems with a newer set of modems operating at higher maximum TDMA symbol rates than older modems can share the same PHYSICAL channel without the need for any change to the older modems or requiring them to transmit on a different frequency. Likewise, TDMA modems can coexist with another set of modems sharing the same PHYSICAL channel using SCDMA multiplexing. Further, two different sets of modems having different symbol rates but both using SCDMA multiplexing can all transmit on the same PHYSICAL channel using the teachings of the invention. The fundamental idea is to use time division multiplexing of burst intervals as between different upstream channels having either the same multiplexing type but different symbol rates or having different multiplexing type and the same or different symbol rates. The invention is not limited to DOCSIS type modems and is not limited to TDMA only or SCDMA only logical channels. The invention is broadly applicable to different upstream channels having any different multiplexing type (such as TDMA, SCDMA, discrete multitone, i.e., OFDM, etc. or the same multiplexing type and different symbol rates).
This end can be achieved without requiring any changes to the older modems to be mixed with newer modems having faster symbol rates or different multiplexing types, or both. More importantly, this end can be achieved without requiring that the older modems operate in a different PHYSICAL channel from the newer modems operating at higher symbol rates or using a different form of multiplexing. More specifically, this end is achieved by providing a CMTS central transceiver with a special backward compatible scheduling process in a distributed digital data communication system. Although all the examples given in this patent application use DOCSIS 1.0 and advanced PHY modems as remote units, those skilled in the art should understand that the process of the invention is generally applicable to any digital data delivery system where multiple upstream logical channels of either different multiplexing types or different symbol rates are sharing the same center frequency or PHYSICAL channel. DOCSIS 1.2 modems are transceivers capable of being configured to transmit either time division multiplexed bursts at configurable symbol rates including symbol rates faster than DOCSIS 1.0 modems, or synchronous code division multiplexed bursts at configurable symbol rates. Modems or transceivers capable of these configuration options are referred to in the claims simply as DOCSIS 1.2 modems since a publicly available DOCSIS 1.2 specification does not currently exist.
A distributed system in which the process can be practiced will have both TDMA only remote transceivers and other remote transceivers. The other remote transceivers may be capable of either SCDMA or TDMA but are programmed to transmit in only SCDMA or TDMA. In some embodiments, the other remote transceivers may be capable of SCDMA only or TDMA at a higher maximum symbol rate than the TDMA only remote transceivers.
The unique scheduling process of the CMTS establishes multiple upstream logical channels that can share the same physical channel. This is accomplished by sending separate downstream messages that create the separate logical channels and schedule separate, nonoverlapping, variable or fixed size TDMA only and SCDMA only time intervals or separate, nonoverlapping, variable or fixed size TDMA at rate 1 and TDMA at rate 2 time intervals. These various logical channels will be referred to as TDMA and SCDMA logical channels, and the TDMA only and SCDMA only or TDMA at different rates intervals will be referred to as regions. One or more TDMA logical channels at different symbol rates and one SCDMA logical channel can share each physical frequency channel.
In the preferred embodiment, DOCSIS 1.0 TDMA only modems can coexist without any change with advanced PHY TDMA or SCDMA modems with the advanced PHY modems operating in TDMA mode bursting out at higher maximum symbol rates than the 1.0 modems. All advanced PHY remote transceivers or RUs programmed by the CMTS to transmit TMA bursts and all DOCSIS 1.0 RUs which transmit only TDMA bursts at the same symbol rate as the 1.2 TDMA modems are assigned to TDMA only logical channel and allowed to transmit only during TDMA regions thereof. If the 1.0 and 1.2 modems are operating at different TDMA symbol rates, they are each assigned to different TDMA logical channels and allowed to transmit only during TDMA regions of those channels. Likewise, all advanced PHY RUs programmed by the CMTS to transmit SCDMA bursts are allowed to transmit only during SCDMA regions of whatever SCDMA logical channel to which they are assigned.
For the DOCSIS type cable modem systems, the CMTS generates a separate UCD and MAP (the MAP message is the message which contains data defining minislots which have been granted by the CMTS) message for each logical channel. In other systems, where nonDOCSIS modems are in use and the media is not necessarily a cable television system, the logical channels are defined by channel descriptor messages which define the characteristics of the logical channels such as center frequency, symbol rate, modulation or multiplexing type, modems that are assigned to the channel, operating mode for the modems, etc. In these alternative embodiments, the channel descriptor messages replace the UCD messages of the DOCSIS type embodiments. Likewise, in these other nonDOCSIS embodiments, the MAP message is replaced with a bandwidth award and scheduling message which defines which transmitters may transmit and when they may transmit.
In the DOCSIS type embodiments, the UCD message creates the logical channel and defines its characteristics, and the MAP message are bandwidth awards and scheduling messages which define the modems which may transmit and the intervals or regions when transmissions are allowed and other silent regions between the transmission regions. The scheduler process in the CMTS allocates only SCDMA bursts in the SCDMA regions and only TDMA bursts in the TDMA regions via separate MAP messages. The MAP messages establish the SCDMA and TDMA regions in the logical channels sharing the same PHYSICAL channel so that TDMA bursts do not overlap SCDMA bursts and TDMA bursts at different symbol rates do not overlap. The MAP message for the TDMA regions defines the time boundaries comprising the start and end of the TDMA region in terms of minislot offset numbers relative to a reference minislot number of the minislot counter in the CMTS for the TDMA only logical channel to which the MAP message pertains. All TDMA bursts must occur only in a TDMA region of a logical channel devoted to TDMA bursts at a specific symbol rate regardless of whether they originate from a DOCSIS 1.0 or an advanced PHY modem. The DOCSIS 1.0 and advanced PHY modems operating to transmit TDMA bursts (assuming they are bursting at the same symbol rate) use their local minislot counters and the SID (a service identification number which is assigned to each modem although DOCSIS modems may have more than one SID although no two modems will have the same SID) and minislot offset numbers in the MAP message to determine when to transmit their data in the interval referred to herein as the TDMA region. NULL SIDS in bandwidth grant awards that do not belong to any RU assigned to a particular TDMA only logical channel define the minislot numbers of silent intervals during which no TDMA modem is allowed to transmit. These silent intervals overlap the SCMDA regions of the SCDMA only logical channel that shares the same physical channel. When two sets of TDMA modems are transmitting at different symbol rates, the system works the same way, but each set of modems transmitting at the same TDMA symbol rate are assigned to one TDMA only logical channel and the other set of modems transmitting at a different symbol rate are assigned to a different TDMA only logical channel.
Likewise, the MAP message for the logical channel used by the advanced PHY modems transmitting SCDMA bursts defines the start and finish times of each SCDMA region in terms of minislot numbers also. Again, a NULL SID not belonging to any RU assigned to the SCDMA only logical channel coupled with a minislot offset in the MAP message defines the time boundaries of one or more silent intervals that overlap the TDMA regions in the TDMA only logical channel or channels that share the same bandwidth. All SCDMA bursts must be within an SCDMA region, and all TDMA bursts must be within a TDMA region of a TDMA only logical channel devoted to TDMA bursts at that symbol rate. TDMA bursts at different symbol rates are transmitted on different logical channels even they may be actually transmitted on the same carrier, i.e., physical channel. No TDMA region of any logical channel can overlap in time with an SCDMA region of another logical channel that shares any of the same bandwidth. Bandwidth is defined as the range of frequencies around the center frequency of the physical channel upon which said upstream logical channel is transmitted from a low frequency rolloff of the Fourier spectrum of said logical channel to a high frequency rolloff of the Fourier spectrum of said logical channel. Likewise, no TDMA region of a first logical channel at a first symbol rate can overlap in time with a TDMA region of a second logical channel at a second symbol rate if any portion of the bandwidths of the first and second logical channels overlap fully or partially. However, a TDMA region at a first symbol rate can overlap in time with a TDMA region at a second symbol rate (which can be the same symbol rate as the first symbol rate) so long as the two TDMA bursts are transmitted on different physical channel carriers having center frequencies that are separated far enough in frequency such that, considering the bandwidths caused by the symbol rates of the first and second TDMA bursts, the bandwidths of the two TDMA only logical channels do not overlap (see the two 3.2 MHz TDMA logical channels on two different physical channels with non overlapping bandwidth in FIG. 6). The CMTS scheduler insures this discipline by writing the SCDMA logical channel MAP message for each SCDMA logical channel such that grants are only issued to the NULL SID during any TDMA regions in any TDMA logical channels sharing the same physical channel. Likewise, the CMTS constructs the MAP messages for each different TDMA logical channel sharing the same physical channel with any other SCMDA only logical channel or a TDMA logical channel at a different symbol rate so as to use the NULL SID to define silent regions during the SCDMA regions or TDMA regions at a different symbol rate of other logical channels. The CMTS scheduler also defines the different logical channels of SCDMA and TDMA bursts at various symbol rates such that these logical channels can share and maximize the use of the original bandwidth assigned to TDMA only modems.